This invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube which alleviates the degradation of color purity due to the deformation of the shadow mask and facilitates the formation of the shadow mask.
Generally, a color cathode ray tube has a vacuum envelope composed of an almost rectangular panel 3 with a curved-surface effective region 1 on whose periphery a sidewall section 2 is provided, and a funnel 4 connected to the sidewall section 2 is provided, and a funnel 4 connected to the sidewall section 2. On the inside face of the effective region 1 of the panel 3, a fluorescent screen 5 is provided. The fluorescent screen 5 is composed of a black non-emitting layers and three-color fluorescent layers formed so as to fill up the spacing of the black non-emitting layer. Inside the fluorescent screen 5, an almost rectangular shadow mask 6 is provided on a mask frame, so as to face the fluorescent screen 5. The shadow mask 6 is supported in a detachable manner by elastic supports 8 hooked on stud pins 7 provided on the sidewall section 2 of the panel 3. In the neck 10 of the funnel 4, an electron gun 12 that emits three electron beams 11B, 11G, 11R is provided. The three electron beams 11B, 11G, 11R emitted from the electron gun 12 are deflected by a deflection unit 13 provided on the outside of the funnel 4 and directed toward the fluorescent screen 5 through the shadow mask 6. The three electron beams 11B, 11G, 11R scan the fluorescent screen 5 horizontally and vertically, thereby producing color images on the fluorescent screen 5.
The shadow mask 6 has the function of selecting the three electron beams 11B, 11G, 11R for the three-color fluorescent layers constituting the fluorescent screen 5 and landing the three electron beams 118, 11G, 11R on the corresponding three-color fluorescent layers. The shadow mask 6 is composed of an almost rectangular mask body 18 and a mask frame 20. As shown in FIGS. 2A and 2B, the mask body 18 is composed of an effective surface 15, a non-aperture section 16 enclosing the effective surface 15, and a bent skirt section 17 formed on the periphery of the non-aperture section 16 which has no through holes. The effective surface 15 is a curved surface, has a large number of electron beam through holes or apertures formed therein, for allowing the electron beams therethrough, and faces the fluorescent screen 5. The mask frame 20 has a sidewall section 19 provided on the skirt section 17 of the mask body 18.
In general, to display an image with a high color purity on the fluorescent screen 5 of the color cathode ray tube, the three electron beams 11B, 11G, 11R have to be selected by the electron beam through holes in the mask body 18 so that the three electron beams 11B, 11G, 11R may land on the three-color fluorescent layers properly. To do this, the positional relationship between the panel and shadow mask 6 needs to be maintained properly. The clearance (q value) between the inside face of the effective region 1 of the panel 3 and the effective surface 15 of the mask body 18 particularly has to be kept within specific permitted limits.
In recent years, to improve the visibility of color cathode ray tubes, effort has been directed toward making the radius of curvature of the outside face of the effective region 1 of the panel larger to bring the outside face close to a plane. In such a panel, the radius of curvature of the inside face of the effective region 1 also needs to be made larger from the viewpoints of the formation of the panel 3 and the visibility. With an increase in the radius of curvature of the inside face of the effective region 1, the radius of curvature of the effective surface 15 of the shadow mask 6 also needs to be made larger to achieve a suitable beam landing.
However, as the radius of curvature of the effective surface 15 of the shadow mask 6 is made larger, the strength with which the curved surface is retained decreases. As a result, a local deformation in manufacturing the shadow mask 6 or a thermal deformation in manufacturing the color cathode ray tube is liable to take place. This may cause a shift in the beam landing, making color purity liable to deteriorate. When the color cathode ray tube has been incorporated into a television set, the sound from the speaker is liable to cause the shadow mask 6 to resonate. The resonance may cause the deterioration (howling) of color purity.
To improve the strength with which the curved surface of the shadow mask 6 is retained, a method of providing a reinforcing bead on the effective surface has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-161306.
However, when a reinforcing bead has been provided on the effective surface with a larger radius of curvature to produce a sufficient curved-surface retaining strength, the spacing between the inside face of the effective region of the panel and the effective surface of the shadow mask falls locally away from the permitted limits. This permits an image of a step caused by the formation of the reinforcing bead to appear on the screen, degrading the picture quality seriously. For this reason, there is a limit to the height of the reinforcing bead. Usually, the limit ranges from about 0.1 to 0.2 mm, which causes the problem of being unable to make the curved-surface retaining strength sufficiently high.
As described above, to improve the visibility of a color cathode ray tube, the radius of curvature of the outside face of the effective region of the panel is made larger to bring the outside face close to a plane. This also requires the radius of curvature of the inside face of the effective region to be made larger. As a result, the radius of curvature of the effective surface of the shadow mask has to be made larger. Making the radius of curvature of the effective surface larger decreases the curved-surface retaining strength.
As a result, a local deformation in manufacturing a shadow mask or a thermal deformation in manufacturing a color cathode ray tube is liable to take place. This may cause a shift in the beam landing and the deterioration of color purity. When the color cathode ray tube has been incorporated into a television set, the sound from the speaker is liable to cause the shadow mask to resonate. The resonance may degrade the color purity.
To improve the strength with which the effective surface of the mask body with a larger radius of curvature is retained, a method of providing a reinforcing bead on the effective surface has been proposed. When a reinforcing bead has been provided on the effective surface with a larger radius of curvature to produce a sufficient curved-surface retaining strength, the spacing between the inside face of the effective region of the panel and the effective surface of the mask body falls locally away from the permitted limits. This permits an image of a step caused by the formation of the reinforcing bead to appear on the screen, seriously degrading the picture quality. For this reason, the curved-surface retaining strength cannot be made sufficiently high by use of a reinforcing bead in a shadow mask whose effective surface has a larger radius of curvature.